Trimethylolpropane (TMP) is a white crystalline material at room temperature, and widely used as starting material in various applications such as alkyd resin, saturated polyester, synthetic lubricant oil, polyurethane resin, plasticizer and the like. Accordingly, there have been continuous researches for economically producing TMP that is an industrially important source material.
TMP is synthesized from n-butyl aldehyde (n-BAL), formaldehyde, and alkali metal hydroxide through aldol condensation reaction and Cannizzaro reaction as represented by the following reaction schemes 1 and 2:
Reaction Scheme 1: Aldol condensation reactionCH3CH2CH2CHO+2HCHO→CH3CH2C(CH2OH)2CHO
Reaction Scheme 2: Cannizzaro reactionCH3CH2C(CH2OH)2CHO+HCHO+NaOH→CH3CH2C(CH2OH)3+HCOONa 
U.S. Pat. No. 3,183,274 discloses a process for producing TMP wherein an excess amount of formaldehyde is used to increase the production yield, and a concentration step is incorporated before the extraction step to reduce the amount of extraction solvent. However, the '274 process requires an additional process for recovering formaldehyde remaining after the reaction since formaldehyde is excessively used to increase the production yield. Also, in the '274 process, a large amount of extraction solvent, about 7 times as much as the amount of raw material, is used to increase the TMP extraction efficiency from a mixture resulted from the synthesis reactions. Thus, the '274 process was disadvantageous in that the costs for facilities to recover the solvent are increased, whereby the investment costs and operational costs are increased. Further, in the '274 process, when extracting TMP by using organic solvent from a mixture resulted from the reaction despite using excessive amounts of formaldehyde and water, the resultant organic extract of TMP contains a considerable amount of sodium formate. Thus, this process has a problem that discoloration of TMP may be caused when the organic extract of TMP was distilled under vacuum.
Also, U.S. Pat. No. 3,956,306 discloses a method for separating TMP from a resultant reaction mixture by using two extraction solvents. However, the '306 method requires additional two separation steps for recovering the two extraction solvents used in the extraction step after the extraction is completed. Further, the amount of alkali metals present in the finally purified TMP is too high by about 42 ppm.
Moreover, U.S. Pat. No. 4,594,461 discloses a method for synthesizing TMP by synthesizing dimethylolbutanal as intermediate by using trialkylamine catalyst, and hydrogenating it by using Pb catalyst. However, the '461 method requires high temperature and high pressure, and thus the costs for facilities satisfying the reaction conditions are high. Further, the yield is decreased when amine is used as catalyst over a long period of time, and so periodic change of the catalyst is required to obtain an appropriately stable yield, whereby the cost for such change is additionally needed.
As described above, the conventional methods for producing TMP have used an excessive amount of starting material to increase the reaction yield, or tried to use various extraction solvents or conditions to separate TMP from alkali metal salt generated during the reaction. As a result, a considerable amount of TMP is lost during the extraction step, and the scale of facilities for extraction and recovery cannot but be increased due to the excessive use of extraction solvent, and thus the conventional methods have had limited commercial application. Further, in the conventional methods as described above, the amount of alkali metal remaining in the resultant even after the alkali metal salt-removing step is as considerably high as 40 ppm or more, and the remaining alkali metal may cause decomposition of TMP by high temperature during a subsequent vacuum distillation step for purifying TMP to highly purified product, resulting in discoloration of the final product.